Traditional IP networks can provide customers with Best-Effort (BE) services only, because all the traffic competes equally for network resources. With the development of new Internet applications, such as voice, video and web services, the desire for quality of service (QoS) becomes stronger.
Recently, the Internet Engineering Task Force (IETF) has proposed a differentiated service (DS) architecture, as described for example in the IETF specification RFC 2475, to provide scalable means to deliver IP quality of service based on handling traffic aggregates, i.e. collections of data packets with the same differentiated service (DS) code point crossing a link in a particular direction. The traffic classification state is conveyed by means of IP-layer packet marking using a DS field in the packet header. Data packets are classified and marked to receive a particular per-hop behavior (PHB) on network nodes along their path. The PHB corresponds to the externally observable forwarding behavior applied at a DS-compliant node to a DS behavior aggregate. Sophisticated classification and traffic conditioning, including marking, policing, and shaping operations, need only be implemented at network boundaries. Within the DS domain, core router forward data packets according to the differentiated services code point (DSCP) value in the packet header.
Up to now, IETF has only defined two kinds of PHBs, i.e. expedited forwarding (EF) PHB and assured forwarding (AF) PHBs. EF class traffic requires low loss, low latency, low jitter, and assured bandwidth for edge-to-edge services through the DS domain. To achieve this, strict policing and shaping is needed. In contrast thereto, no quantitative requirements are defined for AF class traffic. There are four independent AF classes each comprising three drop precedence levels.
Although four AF classes have been defined, it is still unsolved how to deal with the data packets belonging to different AF classes. However, the performance or fairness of responsive flows, such as TCP (Transmit Control Protocol) packets can be affected by non-responsive flows, such as UDP (User Datagram Protocol) packets, if they are marked to the same AF class. This may encourage customers to use non-responsive flows, which will be detrimental to TCP traffic. Therefore, fairness between responsive flows and non-responsive flows has been haunting network researchers for many years. Many papers have been published for the negative impacts of an increasing deployment of non-responsive traffic, i.e. non-congestion-controlled traffic, in the Internet. For example, Sally Floyd and Kevin Fall, “Promoting the Use of End-to-End Congestion Control in the Internet”, IEEE ACM Transaction on Networks, 1999, suggest promoting the use of end-to-end congestion control in the Internet. However, the network side cannot rely on customers' corporation completely. Thus, in the architecture of a DS network, this issue still remains and becomes an obstacle for guaranteed QoS.